The answer is that target group health check failures are the most likely cause of the dropped traffic. When a Network Load Balancer’s health checks fail for a specific target, the NLB immediately stops routing traffic to that instance, even though the client IP appears in the access logs. This is because the NLB operates at layer 4 and relies solely on health check status to determine target availability—it does not inspect security groups or NACLs before dropping the flow. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of NLB’s stateless forwarding behavior versus stateful components like security groups, which would only block traffic after the NLB sends it. A common trap is assuming that security groups or NACLs are the first line of defense, but for an NLB, health check failure is the definitive reason traffic never reaches the target. Memory tip: “No health, no wealth”—if the health check fails, the NLB withholds all traffic.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company has a VPC with multiple subnets across Availability Zones. An application uses a Network Load Balancer (NLB) to distribute traffic to instances. The security team notices that traffic from a specific client IP is being dropped. The NLB access logs show the client IP, but the target instances do not receive the traffic. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The target group health checks are failing, so the NLB is not sending traffic to those targets
If health checks fail, the NLB marks the target as unhealthy and stops sending traffic.
B
The target subnet network ACL blocks the client IP
Why wrong: NACLs affect traffic to the instance, but the NLB would still forward traffic unless the target is unhealthy.
C
The target instance security group does not allow traffic from the NLB
Why wrong: If the security group blocked traffic, the NLB would still forward it, but the instance would drop it. The question says the instances do not receive traffic.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The target group health checks are failing, so the NLB is not sending traffic to those targets
NLB target group health checks might be failing, so the NLB does not send traffic to unhealthy targets. Option A is wrong because security groups on targets would drop traffic after NLB sends it. Option B is wrong because NLB does not have security groups. Option C is wrong because NACLs on the target subnet affect traffic to the instance, but the NLB would still send traffic if the target is healthy.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The target group health checks are failing, so the NLB is not sending traffic to those targets
Why this is correct
If health checks fail, the NLB marks the target as unhealthy and stops sending traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The target subnet network ACL blocks the client IP
Why it's wrong here
NACLs affect traffic to the instance, but the NLB would still forward traffic unless the target is unhealthy.
✗
The target instance security group does not allow traffic from the NLB
Why it's wrong here
If the security group blocked traffic, the NLB would still forward it, but the instance would drop it. The question says the instances do not receive traffic.
✗
The NLB security group blocks the client IP
Why it's wrong here
NLBs do not have security groups.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The target group health checks are failing, so the NLB is not sending traffic to those targets — NLB target group health checks might be failing, so the NLB does not send traffic to unhealthy targets. Option A is wrong because security groups on targets would drop traffic after NLB sends it. Option B is wrong because NLB does not have security groups. Option C is wrong because NACLs on the target subnet affect traffic to the instance, but the NLB would still send traffic if the target is healthy.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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